The Puzzle in Sidgwick's Moral Epistemology

There is a hitherto under-recognized puzzle in Sidgwick’s moral epistemology: In “The Establishment of Ethical First Principles,” Sidgwick distinguishes two ( “quite distinct”) kinds of argument: bipartite arguments and criterial arguments. In Methods III, Sidgwick develops a criterial argument for utilitarianism. In general, he thinks of criterial arguments as different from and more powerful than bipartite arguments. But in Methods IV II, Sidgwick considers only bipartite arguments. Why does the criterial argument for utilitarianism disappear in Methods Book IV?